The present invention relates to a process and apparatus for cooling diesel engine exhaust gases in an exhaust gas recirculation system of the type where the hot exhaust gas coming from engine cylinders is first cooled before it is admitted on the intake side with incoming fresh airflow.
An exhaust gas recirculation system which operates according to the above-mentioned process and has a cooling device is known, for example, from German Patent Document DE 40 20 357 A1. It is there suggested to arrange a heat exchanger or a mixing device in the flow direction behind the cleaning system in a suitable area of the exhaust gas pipe. By means of the provided dilution of the hot exhaust gases with cooling air, an effective cooling of the exhaust gases is achieved and thus the occurrence of overheating damages is prevented.
Because of the occurring high exhaust gas temperatures, the heat exchanger parts or driving devices, which come in contact with the hot exhaust gas flow for the cooling of the diesel engine exhaust gases, are made of high-temperature-stable steel types. Furthermore, during an exhaust gas cooling to temperatures below 250.degree. C., a condensation occurs of acid exhaust gas constituents as well as deposits of exhaust gas constituents on the heat exchanger walls which lead to an impairment of the efficiency and finally to a clogging of the heat exchanger. A cleaning of the heat exchanger with respect to the particles deposited on the pipe walls and of such deposits, irrespective of the construction of the heat exchanger, can be carried out only at very high expenditures.
In particular, the acid constituents of the condensate result in corrosive damage to the heat exchanger.
In order to counteract these undesirable contaminations of the heat exchanger, it is known to manufacture the components of the heat exchanger which are in contact with the exhaust gas flow of corrosion-proof metals which are particularly corrosion-resistant with respect to the sulfurous acid which may, for example, be formed. This measure can prevent a contamination of heat exchangers through which an exhaust gas flows which has a temperature of from 500.degree. to 700.degree. C. However, when the exhaust gas temperature falls to temperatures below 250.degree. C., an adhesion tendency of the exhaust gas particles takes place which is as strong as before.
In order to counteract the adhesion tendency in this temperature range of the exhaust gas flow, it is also known to coat the exhaust-gas-carrying pipes of the heat exchanger with a suitable plastic material, or to manufacture the pipes themselves, for example, of polytetrafluoroethylene (PTFE).
Although, by means of the known measures, a contamination of the heat exchanger in the intended temperature range of the exhaust gas flow can, in each case, be effectively prevented, there is the disadvantage that, in actual operational applications of diesel engines, the exhaust gas flow in the exhaust gas recirculation pipe will fluctuate within a temperature range which is so large, depending on the application conditions and the power output of the engine, that it is not covered by any of the known heat exchangers. As a result, in engine operating conditions with an exhaust gas temperature which is outside the application temperature range of the used heat exchanger, the contamination of and the damage to the heat exchanger will be just as high.
It is therefore an object of the present invention to provide a process and an arrangement suitable for implementing this process for the cooling of the exhaust gas which reliably prevent contaminations and damage and can be implemented at reasonable cost.
This object is achieved according to the present invention by providing an arrangement where the cooling of the hot exhaust air flow is carried out in at least two successive steps in respective serially arranged heat exchangers.
A principal advantage of the cooling of the hot exhaust gas flow according to the invention in at least two steps is the resulting division of the overall cooling temperature difference into individual temperature ranges. Since, in these individual ranges, the cooling temperature difference is in each case lower than in the case of the previously used exhaust gas cooling to the starting temperature, the invention provides the possibility of connecting in series several heat exchangers which are each specifically adapted to a defined temperature range in order to thus prevent the formation of contaminations and/or corrosion damages in the heat exchangers.
Because of the flow direction of the exhaust gas flow, the temperature levels of the individual cooling temperature ranges decrease in the flow direction so that the exhaust gas flow is finally cooled to a temperature below 250.degree. C. before it is entered into the last cooling step. In the case of the corresponding selection of the individual temperature ranges, expediently two or several conventional heat exchangers can be connected in series in the exhaust gas recirculation flow, which results in an extremely simple and therefor low-cost construction of the arrangement according to the invention.
From the large number of known heat exchangers, heat exchangers seem particularly suitable which, with respect to the material and/or coating, are each adapted to an application in a defined temperature range. Preferred materials of such heat exchangers are high-temperature stable steel types or titanium as well as parts made of these materials which are coated with suitable plastic materials.
Basically, a series connection of a plurality of heat exchangers is conceivable which each counteract individual contamination tendencies in the corresponding cooling temperature ranges, which are preferably small temperature ranges.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.